1. Field of the Invention
The present invention relates to a high voltage generating device for outputting a high voltage, and more particularly, to a high voltage generating device configured to raise a high voltage to a target voltage at a high speed and an image forming apparatus including the high voltage generating device.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a surface of an electrophotographic photosensitive member (hereinafter, referred to as a “photosensitive drum”) is uniformly charged by a charging device, and the surface of the charged photosensitive drum is exposed by an exposing device, thus forming an electrostatic latent image on the surface of the photosensitive drum. The electrostatic latent image is developed by a developing device with a developer (hereinafter, referred to as “toner”) to form a toner image, and the developed toner image is transferred to a recording material by a transfer device. The toner image is fixed to the recording material by a fixing device, and then the recording material on which an image is thus formed is output. A transfer roller is used as the transfer device, which forms a nip portion with the photosensitive drum and conveys the recording material through the nip portion. A high voltage of an opposite polarity with respect to the toner (hereinafter, referred to as a “transfer voltage”) is applied to the transfer roller, by which the toner image is transferred to the recording material.
The resistance of the transfer roller is likely to fluctuate in accordance with the ambient temperature and humidity, and if a desired transfer current value is not obtained, an image defect such as a transfer failure or a ghost may occur. The following control is performed to reduce the transfer failure or the image defect. That is, in order to optimize the transfer voltage to be applied to the transfer roller, a resistance value of the transfer roller is measured, and the transfer voltage is controlled in an appropriate manner based on a result of the measurement. This control method is a known control method referred to as an active transfer voltage control (ATVC). By performing this control, the transfer voltage can be applied to maintain an applied current value to an appropriate value even when the impedance of the transfer roller fluctuates due to a change of the environment.
As to the ATVC control, in these days, a method of executing the control by software in a controller has become a mainstream instead of a control by hardware. This is an effective method for achieving simplified and stable circuit configuration and control. Specifically, the transfer voltage is applied to the transfer roller as a constant voltage, and the applied current value detected by hardware is monitored by a controller. A process of obtaining the transfer voltage (voltage value) to be applied is executed by software based on the monitored current value and a target current value. However, if an output range of the transfer voltage and a range of a load fluctuation are broad, the method of executing the control by the software may cause the following problems. For example, if the characteristic of an applied voltage at the time of start-up is greatly changed due to a load condition (such as a load fluctuation), a fluctuation of a start time until the transfer voltage converges to the target voltage may occur, or an overshoot or an undershoot may occur. This may cause a degradation of the image quality or a deterioration of the photosensitive drum.
In a disclosure of Japanese Patent Application Laid-Open No. 2004-088965, an average value of the impedance is calculated from an output value obtained by performing an A/D conversion multiple times from an edge of the recording material and an output voltage obtained by a feedback. A value of a PWM signal (on-duty width) is calculated based on two conditions including a range of the calculated average value (first condition) and a range of a difference between the current output value and the target current value (second condition). According to the disclosure of Japanese Patent Application Laid-Open No. 2004-088965, a convergence time to a desired transfer voltage by a control of the software can be shortened, and the overshoot or the undershoot can be reduced.
As another example of raising the high voltage to the target voltage at a high speed, the following configuration is disclosed in Japanese Patent Application Laid-Open No. H09-093920. That is, there is proposed a method involving comparing a detection voltage of a voltage detecting circuit with a second reference voltage slightly lower than a reference voltage, and if the detection voltage of the voltage detecting circuit exceeds the second reference voltage, controlling a speed of charging a capacitor, which is a load, to be slower. In the disclosure of Japanese Patent Application Laid-Open No. H09-093920, a rapid charging area, a slow charging area, and a hold charging area are provided in order from the time of start-up, and upon starting the start-up, the voltage is rapidly raised by setting the on-duty width of a PWM signal to the maximum on-duty width. When an output voltage reaches the second reference voltage value (about 90% as an example), it is switched to the slow charging area. An integrating circuit is provided on an input side of a circuit for generating a pulse of the PWM signal, the rapid charging is performed at an initial stage of the rising by the integrating circuit. After that, the overshoot and the undershoot are suppressed by slightly charging and discharging in the slow charging area and the hold charge area.
An effort is being made to achieve a high-speed control of the transfer voltage and to reduce the overshoot and the undershoot. In these days, it is demanded to further shorten a time period from a time when a print instruction is issued (a print command is sent) from a computer or the like till a time when a printing on the first recording material is completed, to enhance a productivity of the image forming apparatus. Hereinafter, the time period from the issuance of the print instruction till the completion of the printing on the first recording material is referred to as a first print out time (FPOT). By shortening the FPOT, a user can achieve a merit that the printing is completed in a short time since the print instruction is issued. In order to shorten the FPOT further, a time required for the ATVC control needs to be further shortened.
The control method of converging to the target voltage by the software, which is described in Japanese Patent Application Laid-Open No. 2004-088965, provides an effect of shortening the time to some extent. However, because a setting update by the software is executed at a certain interval, a control cycle is relatively long. In addition, a convergence time is needed for an accumulated number of updates. For this reason, there is a limit in the control by the software for the purpose of convergence to the target voltage in a shorter period of time. In the control method described in Japanese Patent Application Laid-Open No. 2004-088965, the convergence to the target voltage is performed with an open loop control by changing an on-duty width of a PWM signal for switching booster transformers. In this control method, an output value is detected after waiting a start-up of the hardware (reaching a steady area), and then the next setting value is updated. It can achieve a high speed if there is a characteristic in which the on-duty width and the output voltage (a value reached in the steady area without a feedback control) have a linear relationship therebetween. However, it is not easy to construct a circuit having a linear characteristic, and it is hard to maintain the linearity due to an influence of fluctuation in a time constant of the circuit and each element. If the linearity cannot be maintained, a difference occurs in a change amount of the output voltage even with the change amount in the same time width, resulting in degradations of the stability and the accuracy of the output voltage control. On the other hand, if an attempt is made to enhance the linearity, another problem is likely to occur conversely, such as a degradation of responsiveness.
In the disclosure of Japanese Patent Application Laid-Open No. H09-093920, a control of maintaining the output voltage to the target voltage by slightly increasing or decreasing the output voltage with a slight increase or decrease of an input voltage is performed with respect to a circuit for outputting the pulse of the PWM signal in the hold charging area in which a control of holding the target voltage is executed. However, when making a transition from the slow charging area to the hold charging area, the input voltage is only decreased by a considerably small amount, and hence it is hard to reduce the overshoot voltage. In order to reduce the overshoot voltage, the rise of the voltage in the slow charging area needs to be even slower. However, if the rise of the voltage is too slow, it will cause a long rise time. In addition, if the circuit is configured to use an integrating circuit on the input side of the circuit for outputting the PWM pulse, it takes a start-up time (integrating time) when the on-duty width of the PWM signal rises from zero to the maximum on-duty width.